Approximate Analytical Solutions of Systems of Pdes by Homotopy Analysis Method

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Computers and Mathematics with Applications 55 (2008) 2913–2923 www.elsevier.com/locate/camwa

Approximate analytical solutions of systems of PDEs by homotopy analysis method

A. Sami Bataineh, M.S.M. Noorani, I. Hashim∗

School of Mathematical Sciences, Universiti Kebangsaan Malaysia, 43600 UKM Bangi Selangor, Malaysia

Received 24 May 2007; received in revised form 2 November 2007; accepted 17 November 2007

Abstract

In this paper, the homotopy analysis method (HAM) is applied to obtain series solutions to linear and nonlinear systems of ﬁrst- and second-order partial differential equations (PDEs). The HAM solutions contain an auxiliary parameter which provides a convenient way of controlling the convergence region of series solutions. It is shown in particular that the solutions obtained by the variational iteration method (VIM) are only special cases of the HAM solutions. c 2007 Elsevier Ltd. All rights reserved.

Keywords: Homotopy analysis method; Systems of PDEs; Nonlinear systems; Variational iteration method; Adomian decomposition method

1. Introduction

The homotopy analysis method (HAM) was first proposed by Liao in his Ph.D. thesis [1]. A systematic and clear exposition on HAM is given in [2]. In recent years, this method has been successfully employed to solve many types of nonlinear, homogeneous or nonhomogeneous, equations and systems of equations as well as problems in science and engineering [3–23]. Very recently, Ahmad Bataineh et al. [24,25] presented two modifications of HAM to solve linear and nonlinear ODEs. The HAM contains a certain auxiliary parameter h¯ which provides us with a simple way to adjust and control the convergence region and rate of convergence of the series solution. Moreover, by means of the so-called h¯-curve, it is easy to determine the valid regions of h¯ to gain a convergent series solution. Thus, through HAM, explicit analytic solutions of nonlinear problems are possible. Systems of partial differential equations (PDEs) arise in many scientific models such as the propagation of shallow water waves and the Brusselator model of the chemical reaction-diffusion model. Very recently, Batiha et al. [26] improved Wazwaz’s [27] results on the application of the variational iteration method (VIM) to solve some linear and nonlinear systems of PDEs. In [28], Saha Ray implemented the modified Adomian decomposition method (ADM) for solving the coupled sine-Gordon equation. In this paper, we present an alternative approach based on HAM to approximate the solutions of linear and nonlinear systems of first- and second-order PDEs. Comparisons with the solutions obtained by VIM [27] and ADM [28] shall be made. It is shown in particular that the VIM [27] solutions are just special cases of the HAM solutions.

∗ Corresponding author. E-mail address: ishak [email protected] (I. Hashim).

2. Basic ideas of HAM

We consider the following differential equations,

Ni [zi (x, t)] = 0, i = 1, 2,..., n, where Ni are nonlinear operators that represent the whole equations, x and t denote the independent variables and zi (x, t) are unknowns function respectively. By means of generalizing the traditional homotopy method, Liao [2] constructed the so-called zero-order deformation equations

(1 − q)L[φi (x, t; q) − zi,0(x, t)] = q h¯ i Ni [φi (x, t; q)], (1) where q ∈ [0, 1] is an embedding parameter, h¯ i are nonzero auxiliary functions, L is an auxiliary linear operator, zi,0(x, t) are initial guesses of zi (x, t) and φi (x, t; q) are unknown functions. It is important to note that, one has great freedom to choose auxiliary objects such as h¯ i and L in HAM. Obviously, when q = 0 and q = 1, both

φi (x, t; 0) = zi,0(x, t) and φi (x, t; 1) = zi (x, t), hold. Thus as q increases from 0 to 1, the solutions φi (x, t; q) varies from the initial guesses zi,0(x, t) to the solutions zi (x, t). Expanding φi (x, t; q) in Taylor series with respect to q, one has

+∞ X m φi (x, t; q) = zi,0(x, t) + zi,m(x, t)q , (2) m=1 where 1 ∂mφ (x, t; q) = i zi,m m . (3) m! ∂q q=0

If the auxiliary linear operator, the initial guesses, the auxiliary parameters h¯ i , and the auxiliary functions are properly chosen, then the series equation (2) converges at q = 1 and +∞ X φi (x, t; 1) = zi,0(x, t) + zi,m(x, t), m=1 which must be one of solutions of the original nonlinear equations, as proved by Liao [2]. As h¯ i = −1, Eq. (1) becomes

(1 − q)L[φi (x, t; q) − zi,0(x, t)] + q Ni [φi (x, t; q)] = 0, (4) which are mostly used in the homotopy-perturbation method [29]. According to (3), the governing equations can be deduced from the zero-order deformation equations (1). Deﬁne the vectors

Ezi,n = {zi,0(x, t), zi,1(x, t), . . . , zi,n(x, t)}. Differentiating (1) m times with respect to the embedding parameter q and then setting q = 0 and ﬁnally dividing them by m!, we have the so-called mth-order deformation equations

L[zi,m(x, t) − χm zi,m−1(x, t)] = h¯ i Ri,m(Ezi,m−1), (5) where 1 ∂m−1 N [φ (x, t; q)] R (Ez ) = i i , i,m i,m−1 m−1 (6) (m − 1)! ∂q q=0 and 0, m ≤ 1, χ = m 1, m > 1. A. Sami Bataineh et al. / Computers and Mathematics with Applications 55 (2008) 2913–2923 2915

It should be emphasized that zi,m(x, t) (m ≥ 1) is governed by the linear equation (5) with the linear boundary conditions that come from the original problem, which can be easily solved by symbolic computation softwares such as Maple and Mathematica.

3. Applications

We will apply the HAM to linear and nonlinear systems of PDEs to illustrate the strength of the method and to establish exact and/or approximate solutions for these problems. Comparisons with the VIM [27] and ADM [28] shall be made.

3.1. Homogeneous linear system

First we present the analytical solutions for the linear homogeneous system of PDEs:

ut − vx + (u + v) = 0, (7) vt − ux + (u + v) = 0, (8) subject to the initial conditions u(x, 0) = sinh x, v(x, 0) = cosh x. (9) To solve system (7)–(9) by means of homotopy analysis method HAM, we choose the initial approximations

u0(x, t) = sinh x, v0(x, t) = cosh x, and the linear operator

∂φi (x, t; q) L[φi (x, t; q)] = , i = 1, 2, (10) ∂t with the property

L[ci ] = 0, (11) where ci (i = 1, 2) are integral constants. Furthermore, systems (7) and (8) suggest that we deﬁne a system of nonlinear operators as

∂φ1(x, t; q) ∂φ2(x, t; q) N [φi (x, t; q)] = − + φ (x, t; q) + φ (x, t; q), 1 ∂t ∂x 1 2 ∂φ2(x, t; q) ∂φ1(x, t; q) N [φi (x, t; q)] = − + φ (x, t; q) + φ (x, t; q). 2 ∂t ∂x 1 2 Using the above deﬁnition, we construct the zeroth-order deformation equations (1 − q)L φi (x, t; q) − zi,0(x, t) = q h¯ i Ni [φi (x, t; q)] , i = 1, 2. (12) Obviously, when q = 0 and q = 1,

φ1(x, t; 0) = z1,0(x, t) = u0(x, t), φ1(x, t; 1) = u(x, t), φ2(x, t; 0) = z2,0(x, t) = v0(x, t), φ2(x, t; 1) = v(x, t).

Therefore, as the embedding parameter q increases from 0 to 1, φi (x, t; q) varies from the initial guess zi,0(x, t) to the solution zi (x, t) for i = 1, 2. Expanding φi (x, t; q) in Taylor series with respect to q one has +∞ X m φi (x, t; q) = zi,0(x, t) + zi,m(x, t)q , m=1 where 1 ∂mφ (x, t; q) = i zi,m(x, t) m . m! ∂q q=0 2916 A. Sami Bataineh et al. / Computers and Mathematics with Applications 55 (2008) 2913–2923

If the auxiliary linear operator, the initial guesses and the auxiliary parameters h¯ i are properly chosen, the above series is convergent at q = 1, then one has +∞ X u(x, t) = z1,0(x, t) + z1,m(x, t), m=1 +∞ X u(x, t) = z2,0(x, t) + z2,m(x, t), m=1 which must be one of the solutions of the original nonlinear equations, as proved by Liao [2]. Now we deﬁne the vector

Ezi,n = {zi,0(x, t), zi,1(x, t), . . . , zi,n(x, t)}. So the mth-order deformation equations is L zi,m(x, t) − χm zi,m−1(x, t) = h¯ i Ri,m Ezi,m−1 , (13) with the initial conditions

zi,m(x, 0) = 0, (14) where

R1,m(Ezi,m−1) = (z1,m−1)t − (z2,m−1)x + z1,m−1 + z2,m−1, R2,m(Ezi,m−1) = (z2,m−1)t − (z1,m−1)x + z1,m−1 + z2,m−1. Now, the solution of the mth-order deformation equation (13) for m ≥ 1 becomes Z t zi,m(x, t) = χm zi,m−1(x, t) + h¯ i Ri,m(Ezi,m−1)dτ + ci , (15) 0 where the integration constants ci (i = 1, 2) are determined by the initial conditions (14). We now successively obtain

z1,1(x, t) = ht¯ cosh x, (16) ht¯ z (x, t) = [2(1 + h¯) cosh x + ht¯ sinh x], (17) 1,2 2 ht¯ z (x, t) = [(6 + 12h¯ + 6 h¯ 2 + h¯ 2 t2) cosh x + 6ht¯ (1 + h¯) sinh x], (18) 1,3 6 z2,1(x, t) = ht¯ sinh x, (19) ht¯ z (x, t) = [2(1 + h¯) sinh x + ht¯ cosh x], (20) 2,2 2 ht¯ z (x, t) = [(6 + 12h¯ + 6 h¯ 2 + h¯ 2 t2) sinh x + 6ht¯ (1 + h¯) cosh x], (21) 2,3 6 etc. Then the series solutions expression by HAM can be written in the form

u(x, t) = z1,0(x, t) + z1,1(x, t) + z1,2(x, t) + z1,3(x, t) + · · · , (22) v(x, t) = z2,0(x, t) + z2,1(x, t) + z2,2(x, t) + z2,3(x, t) + · · · , (23) or speciﬁcally when h¯ = −1, t2 t4 t3 t5 u(x, t) = sinh x 1 + + + · · · − cosh x t + + + · · · , 2! 4! 3! 5! t2 t4 t3 t5 v(x, t) = cosh x 1 + + + · · · − sinh x t + + + · · · , 2! 4! 3! 5! A. Sami Bataineh et al. / Computers and Mathematics with Applications 55 (2008) 2913–2923 2917 which are exactly the same as the solutions obtained by VIM [27] converging to the closed-form solutions, u(x, t) = sinh(x − t), v(x, t) = cosh(x − t). (24)

3.2. Nonhomogeneous linear system

Now consider the following nonhomogeneous linear system:

ut − vx − (u − v) = −2, (25) vt − ux − (u − v) = −2, (26) subject to the initial conditions u(x, 0) = 1 + ex , v(x, 0) = −1 + ex . (27) To solve system (25)–(27) by means of HAM, we choose the initial approximations x x u0(x, t) = 1 + e , v0(x, t) = −1 + e , and the linear operator as in (10) with the property (11), the zero-order deformation equations (12) and the mth-order deformation equations (13) with the initial conditions (14), where

R1,m(Ezi,m−1) = (z1,m−1)t − (z2,m−1)x − z1,m−1 + z2,m−1 + 2 − 2χm, R1,m(Ezi,m−1) = (z2,m−1)t + (z1,m−1)x − z1,m−1 + z2,m−1 + 2 − 2χm.

Now, the solution of the mth-order deformation for m ≥ 1 are the same as (15), where the integration constants ci (i = 1, 2) are determined by the same initial conditions (14). We now successively obtain x z1,1(x, t) = −ht¯ e , (28) ht¯ ex z (x, t) = [−2 + h¯(t − 2)], (29) 1,2 2 ht¯ ex z (x, t) = − [6 − 6h¯(t − 2) + h¯ 2(t2 − 6t + 6)], (30) 1,3 6 x z2,1(x, t) = ht¯ e , (31) ht¯ ex z (x, t) = [2 + h¯(t + 2)], (32) 2,2 2 ht¯ ex z (x, t) = [6 + 6h¯(t + 2) + h¯ 2(t2 + 6t + 6)], (33) 2,3 6 etc. In the special case h¯ = −1, we recover the VIM solutions [27], t2 t3 u(x, t) = 1 + ex 1 + + + · · · , 2! 3! t2 t3 v(x, t) = 1 + ex 1 − t + − + · · · , 2! 3! which converge to the closed-form solutions u(x, t) = 1 + ex+t , v(x, t) = −1 + ex−t .

3.3. Nonhomogeneous nonlinear system

Consider the following nonhomogeneous nonlinear system:

ut + vux + u − 1 = 0, (34) vt − uvx − v − 1 = 0, (35) 2918 A. Sami Bataineh et al. / Computers and Mathematics with Applications 55 (2008) 2913–2923 subject to the initial conditions

u(x, 0) = ex , v(x, 0) = e−x . (36) To solve system (34)–(36) by means of HAM, we choose the initial approximations x −x u0(x, t) = e , v0(x, t) = e , and the linear operator as in (10) with the property (11), the zero-order deformation equations (12) and the mth-order deformation equations (13) with the initial conditions (14). We now successively obtain x z1,1(x, t) = ht¯ e , (37) ht¯ ex z (x, t) = [2 + h¯(t + 2)], (38) 1,2 2 ht¯ ex z (x, t) = [6 + 6h¯(t + 2) + h¯ 2(t2 + 6t + 6)], (39) 1,3 6 −x z2,1(x, t) = −ht¯ e , (40) ht¯ e−x z (x, t) = [−2 + h¯(t − 2)], (41) 2,2 2 ht¯ e−x z (x, t) = − [6 − 6h¯(t − 2) + h¯ 2(t2 − 6t + 6)], (42) 2,3 6 etc. In the special case h¯ = −1, we recover the VIM solutions [27], t2 t3 u(x, t) = ex 1 − t + − + · · · , 2! 3! t2 t3 v(x, t) = e−x 1 + t + + + · · · , 2! 3! which are convergent to the closed-form solutions, u(x, t) = ex−t , v(x, t) = e−x+t .

3.4. Homogeneous nonlinear system

We consider to examine the homogeneous nonlinear system [30]:

ut + ux vx + u yvy + u = 0, (43) vt + vx wx − vywy − v = 0, (44) wt + wx ux + wyu y − w = 0, (45) subject to the initial conditions

u(x, y, 0) = ex+y, v(x, y, 0) = ex−y, w(x, y, 0) = e−x+y. (46) According to HAM, the initial approximations are selected by using the given initial conditions x+y x−y −x+y u0(x, y, t) = e , v0(x, y, t) = e , w0(x, y, t) = e , and the linear operator

∂φi (x, t; q) L[φi (x, t; q)] = , i = 1, 2, 3, ∂t with the property

L[ci ] = 0, A. Sami Bataineh et al. / Computers and Mathematics with Applications 55 (2008) 2913–2923 2919 where ci (i = 1, 2, 3) are integral constants. Following similar procedure as in the previous subsections, the solution of the mth-order deformation equation for m ≥ 1 becomes Z t zi,m(x, t) = χm zi,m−1(x, t) + h¯ i Ri,m(Ezi,m−1)dτ + ci , (47) 0 where the integration constants ci (i = 1, 2, 3) are determined by the same initial conditions (14). We now successively obtain x+y z1,1(x, y, t) = ht¯ e , (48) ht¯ ex+y z (x, y, t) = [2 + h¯(t + 2)], (49) 1,2 2 ht¯ ex+y z (x, y, t) = [6 + 6h¯(t + 2) + h¯ 2(t2 + 6t + 6)], (50) 1,3 6 x−y z2,1(x, y, t) = −ht¯ e , (51) ht¯ ex−y z (x, y, t) = [−2 + h¯(t − 2)], (52) 2,2 2 ht¯ ex−y z (x, y, t) = − [6 − 6h¯(t − 2) + h¯ 2(t2 − 6t + 6)], (53) 2,3 6 −x+y z3,1(x, y, t) = −ht¯ e , (54) ht¯ e−x+y z (x, y, t) = [−2 + h¯(t − 2)], (55) 3,2 2 ht¯ e−x+y z (x, y, t) = − [6 − 6h¯(t − 2) + h¯ 2(t2 − 6t + 6)], (56) 3,3 6 etc. Then the series solutions expression by HAM can be written in the form

u(x, t) = z1,0(x, t) + z1,1(x, t) + z1,2(x, t) + z1,3(x, t) + · · · (57) v(x, t) = z2,0(x, t) + z2,1(x, t) + z2,2(x, t) + z2,3(x, t) + · · · (58) w(x, t) = z3,0(x, t) + z3,1(x, t) + z3,2(x, t) + z3,3(x, t) + · · · . (59) So in the special case h¯ = −1, we obtain t2 t3 u(x, t) = ex+y 1 − t + − + · · · , 2! 3! t2 t3 v(x, t) = ex−y 1 + t + + + · · · , 2! 3! t2 t3 w(x, t) = e−x+y 1 + t + + + · · · , 2! 3! which are the VIM solutions [27] converging to the closed-form solutions, u(x, t) = ex+y−t , v(x, t) = ex−y+t , v(x, t) = e−x+y+t .

3.5. Coupled sine-Gordon equations

We ﬁnally consider a system of nonlinear second-order PDEs given by the coupled sine-Gordon equations [28]:

2 utt − uxx = −δ sin(u − w), (60) 2 wtt − c wxx = sin(u − w), (61) 2920 A. Sami Bataineh et al. / Computers and Mathematics with Applications 55 (2008) 2913–2923

Fig. 1. The h¯-curve of ut (0, 0) and vtt (0, 0) given by (22) and (23) based on the ﬁfth-order HAM approximations. subject to the initial conditions

u(x, 0) = A cos(kx), ut (x, 0) = 0, (62) w(x, 0) = 0, wt (x, 0) = 0, (63) where c, δ, A and k are constants. To solve (60)–(63) by means of HAM, we choose the initial approximations

u0(x, t) = A cos(kx), w0(x, t) = 0, and the linear operators for i = 1, 2 2 ∂ φi (x, t; q) L[φi (x, t; q)] = , ∂t2 with the property

L[ci + ci t] = 0, where ci (i = 1, 2) are integral constants. Following similar procedure as in the previous examples, we obtain: h¯ z (x, t) = [Ak2 cos kx + δ2 sin(A cos kx)]t2, 1,1 2 h¯ z (x, t) = − sin(A cos kx)t2. 2,1 2 So, the 2-term HAM series solutions in the special case h¯ = −1 are 1 u(x, t) = A cos kx − [Ak2 cos kx + δ2 sin(A cos kx)]t2, (64) 2 1 w(x, t) = sin(A cos kx)t2. (65) 2 The validity of the method is based on such an assumption that the series (2) converges at q = 1. It is the auxiliary parameter h¯ which ensures that this assumption can be satisfied. In general, by means of the so-called h¯-curve, it is straightforward to choose a proper value of h¯ which ensures that the solution series is convergent. The h¯-curves for the five examples considered in this paper are presented in Figs. 1–3 which were obtained based on the fifth- order HAM approximations solutions. By HAM, it is easy to discover the valid region of h¯, which corresponds to the line segments nearly parallel to the horizontal axis. In HAM it possible to obtain a large family of solutions. For the examples considered in this work, the special case h¯ = −1 yields the VIM solutions [27] and hence the exact solutions. Fig. 4 discussed the numerical comparison between the fifth-order HAM with different values of h¯ and the exact solution of the problem in Section 3.1 in the interval x ∈ [0, 1] at t = 1 the result shows that the proper value of h¯ is −1. Figs. 5 and 6 show the comparisons between the 3-term of HAM and the 5-term of ADM solutions [28] for the case h¯ = −1, c = δ = A = 1 and k = 1.6. The results presented in Figs. 5 and 6 clearly show A. Sami Bataineh et al. / Computers and Mathematics with Applications 55 (2008) 2913–2923 2921

Fig. 2. The h¯-curve of ut (0, 0) and vt (0, 0) given by (28)–(33) and (37)–(42) based on the ﬁfth-order HAM approximations.

Fig. 3. The h¯-curve of ut (0, 0, 0), vt (0, 0, 0) and wt (0, 0, 0) given by (57)–(59) based on the ﬁfth-order HAM approximations.

Fig. 4. The ﬁfth-order HAM solutions (22) and (23) for different values of h¯ vs the exact solution (24). the good accuracy of HAM. In addition, HAM avoids the need for calculating the Adomian polynomials which can be complicated.

4. Conclusions

In this paper, it was shown how HAM can be applied to systems of PDEs. By HAM we obtained a family of solutions whose special cases are the solutions obtained by VIM and ADM. The advantage of HAM is the auxiliary parameter which provides a convenient way of controlling the convergence region of series solutions. This is not possible in other analytic methods like VIM and ADM. It is shown that the homotopy analysis method is a promising tool for other more complicated linear or nonlinear, homogeneous or nonhomogeneous, systems of PDEs. 2922 A. Sami Bataineh et al. / Computers and Mathematics with Applications 55 (2008) 2913–2923

Fig. 5. (a) The numerical results for u(x, t) by means of 3-term HAM solution. (b) The numerical results for u(x, t) by means of 5-term ADM solution [28].

Fig. 6. (a) The numerical results for w(x, t) by means of 3-term HAM solution. (b) The numerical results for w(x, t) by means of 5-term ADM solution [28].

Acknowledgments

The authors would like to acknowledge the ﬁnancial support received from the Academy of Sciences Malaysia under the SAGA grant no. P24c (STGL-011-2006). The reviewers are acknowledged for their comments which improved the presentation of the paper. A. Sami Bataineh et al. / Computers and Mathematics with Applications 55 (2008) 2913–2923 2923

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